Decoupling capacitors are used in integrated circuits to electrically decouple one part of the integrated circuit from another part of the integrated circuit. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
Decoupling capacitors are typically formed in a circuit with a resistor, with the resistor and the capacitor in series, such as between the power VDD and ground VSS rails of the integrated circuit. Unfortunately, these combinations of a single capacitor with a single resistor tend to require a lot of surface area, and thus can be difficult to place, especially as the trend to decrease chip size continues.
What is needed, therefore, is a decoupling circuit design that increases the apparent capacitance density, so that—for example—smaller decoupling structures can be placed in the circuit, and yet provide the same capacitance.